save and load solution of a model

[Federico Lozano Santamaria]

Hello,

Is there a way to save the whole solution of a problem, for example in a .sol file or any other format, and then load it as a starting point for the same problem or for another similar one? What I have in mind is something similar to the gdx files of gams.

Federico

[Gabriel Hackebeil]

Python has many modules for taking an object like a dictionary and easily saving / loading that object to / from a file (e.g., json, yaml, pickle).

What you need to do on the Pyomo side is create special component names that Pyomo can use to efficiently find an object on a model when you want to reload the solution. Here is an example (that you can customize):

from pyomo.environ import *
from pyomo.core.base.block import generate_cuid_names

model = ConcreteModel()
model.x = Var()
model.X = Var([1,2])

model.x.value = 2
model.X[1].value = 3
model.X[2].value = 4

# generate cuid names efficiently in bulk                                                                                 
labels = generate_cuid_names(model)

# store the variable values into a dictionary                                                                             
solution = {}
for var in model.component_data_objects(Var):
    solution[labels[var]] = var.value

# save the solution                                                                                                       
import json
with open("junk.json", "w") as f:
    json.dump(solution, f)
del solution

model.x.value = None
model.X[1].value = None
model.X[2].value = None

# load the solution                                                                                                       
with open("junk.json") as f:
    solution = json.load(f)

# store the solution into the variables                                                                                   
for cuid, val in solution.items():
    model.find_component(cuid).value = val

Note that you can modify this to store other properties of a “solution”, such as whether or not a variable has been fixed, constraint duals, etc.

Gabe

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[Federico Lozano Santamaria]

Thanks for your response Gabe